Transparent displays are an increasingly interesting area for such applications as retail advertising and window displays. In particular, the use in a window requires a high level of transparency and the use as a retail display for perishable or light sensitive items (such as food or clothing) also requires a high transparency. A number of technologies have been suggested in order to achieve this aim. For example:
Transparent displays based on LCD or OLED technologies are common but typically suffer from low overall transparency, below 40% in the case of LCDs and 60% for OLEDs. OLEDs are also difficult to make large and bright.
Alternative technologies such as cholesteric LC or switchable scattering screens typically have low viewing angle and low switching speed that limit the potential applications.
There are projection screen based patents such as US2005/0264880 (Kim et al., Dec. 1, 2005) and JP2007/536595 (LG, Dec. 13, 2007) that concern themselves with optics and projector systems for improved ambient contrast, but the screens they describe are opaque. Rear projection TV screens, such as prism structures in JP5343490 (Tetsuya et al., Apr. 15, 2010) are also designed not to be seen through, and therefore are not transparent.
GB 2428307 (Roy et al., Jan. 24, 2007) is a transparent screen based on diffusing light, which is simple to fabricate but has a low clarity of view, in that objects seen through the screen are diffused and not well defined.
JP 2001/005101 (Atsushi et al., Jan. 12, 2001) has a differential scatterer, scattering from one direction (projector) and clear in the other (viewer). The clarity, although better, is generally low. It is also difficult to fabricate.
JP 2005/292679 (Kazunori, Oct. 20, 2005) and U.S. Pat. No. 8,212,744 (Kuo et al., Jan. 24, 2008) have louvre diffuser mirrors that scatter light forward, whereas the screen is transparent when viewed along the louvre. The viewing angle is very restricted here and it is also difficult to fabricate. The form factor of the display is also not suited to some applications, for example a window.
More recent screens, such as CN103050068 (Wang, Apr. 17, 2013) and Holopro, use holographic sheets, which are non-diffusing and not prism based and can redirect light. Another recent screen by Sun Innovations is described in paper 54.4 at the Society of Information Display Week Conference, Vancouver, June 2013. This screen uses tailored RGB nanophosphors on a screen that respond to blue light of slightly different wavelengths. The phosphors do not respond to other wavelengths so appear transparent.
Although highly transparent (above 80%), the efficiency is limited, the holographic displays have some loss of clarity and have colour issues. They are also difficult and expensive to fabricate, especially at large size. The light sensitive material used to make the hologram also limits lifetime and reliability in well lit environments such as an external window
Referring to the above conventional configurations, there is no high transparency transparent screen (>80%) that is simple and cheap to manufacture, with a wide viewing angle and with no colour issues, that has high materials reliability, is able to run at video rates and has high image display efficiency, i.e. a high screen gain for the transparency, i.e. gain/(1−transparency)>>1.